[priya-ni]

This pull request adds support for the has_children field to the Step model in the V2 data store API. The has_children field is output-only and indicates whether a step has ever had a committed child, even if that child was later deleted. The changes include updating the data model, read field enums, protobuf serialization/deserialization, and relevant tests to handle this new field.

Step model enhancements:

• Added the has_children attribute to the Step class, including its constructor, equality comparison, and documentation. This field is marked as output-only and is set based on the corresponding protobuf field. [1] [2] [3] [4] [5] [6]

• Updated the documentation to describe the new Step.has_children field, its semantics, and compatibility notes regarding older generated proto packages.

Read field and enum updates:

• Added HAS_CHILDREN to the StepField enum, allowing clients to request this field when reading steps.

Test coverage improvements:

• Enhanced unit tests to verify reading and omission of the has_children field in protobuf serialization/deserialization, and updated tests to check that HAS_CHILDREN is sent in the read mask when requested. [1] [2] [3]

• Updated the set of output-only init parameters in tests to include has_children.

Test utilities:

• Minor import adjustment in test files for type casting.### What does this Pull Request accomplish?

TODO: Include high-level description of the changes in this pull request.

Why should this Pull Request be merged?

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What testing has been done?

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